Fuel tanks of thermoplastic material are basically to be impermeable to hydrocarbons. They are usually produced from multi-layer co-extrudates containing barrier layers for hydrocarbons. The plastic materials used for the production of plastic fuel tanks, for example polyethylene, are basically not diffusion-tight in relation to hydrocarbons. In addition those plastic materials have a tendency to swell up in the presence of fuel. For that reason, the known fuel tanks of plastic material are provided with a multi-layer wall in which an EVOH layer is embedded as a hydrocarbons barrier. In that respect, the lack of diffusion-tight integrity of the base material used does not represent any problem.
Usually, however, fuel tanks are provided with installation fitments and they have air intake and venting lines as well as electrical connections. To produce means for passing them through the tank wall, it is known for the finished container to be provided with cut-out openings into which connection means, valves or the like are fitted. The valves, valve covers and nipples are welded to the outside wall of the container in such a way as to cover the respective opening. Such connecting locations on a fuel tank represent potential leakage points. The term leakage does not mean the loss of liquid in the true sense, but rather that means diffusion leakage or diffusion losses in the region of seam locations and/or welds without involving additional barrier measures. In order to avoid leakage paths at connections on the fuel tank, it is known for connection means and valves to be produced in the form of complicated and expensive two-component structural parts, in which case those structural parts usually have at least one part comprising a plastic material which is weldable to the tank wall. Functional parts on such elements in contrast usually consist of plastic materials which do not swell up in the presence of fuel. Those plastic materials generally cannot be welded to the plastic materials used for the tank wall.
A process for the production of a fuel tank of thermoplastic material and such a fuel tank having connection elements for hoses or the like is known for example from EP 1 211 196. EP 1 211 196 A1 describes a double-shell fuel tank of thermoplastic material with a barrier layer for hydrocarbons, which is embedded in the container wall, the shell portions which are assembled to constitute the container having been produced by thermoforming. In order to ensure simple mounting of connection elements while maintaining a high level of permeation sealing integrity in relation to hydrocarbons, EP 1 211 196 A1 proposes a two-part connection element or two-part fitting which includes at least an outer component and an inner component, wherein the outer component is welded to the outer wall of the container and the inner component at least partially passes through the outer component with displacement and intermediate positioning of the container wall. That arrangement has the advantage that the container wall is so displaced and enclosed between the component parts that the displaced material serves as a sealing means for the passage through the container wall.
Such a passage through the container wall can be comparatively easily produced in the operation of thermoforming the half-shell portions of the container, but such a connection is more difficult to implement when producing a fuel tank by extrusion blow molding.
Therefore, the object of the invention is to provide a process for the production of a container, in particular a fuel tank of thermoplastic material, with which it is possible for connection elements to be provided on the container during the production thereof using the simplest possible means. In that respect, the invention seeks to provide that as far as possible leakage paths for hydrocarbons in the wall of the container are avoided or are restricted to a minimum.